Method and apparatus for preparing a metal or metal-alloy product for a casting process

ABSTRACT

The present invention relates to a method and apparatus for preparing a metal or metal-alloy product for a casting process—wherein the product is brought into a partly solidified (semi-solidified) state before casting—in which the product contains crystallization nuclei uniformly distributed throughout its volume. The method involves introducing an amount of a chosen alloy (in pulverized form) and an amount of a chosen melt, which is at a temperature above the liquefaction temperature of the alloy, into a crystallization vessel, which is heated to below the liquefaction temperature of the alloy, and mixing the melt and the alloy together in the crystallization vessel by means of electrical and/or magnetic forces to create the desired product.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of German patent application101212349.7-24 filed Mar. 13, 2002, incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a method and apparatus forpreparing a metal or metal-alloy product for a casting process—whereinthe product is brought into a partly solidified (semi-solidified) statebefore casting—in which the product contains crystallization nucleiuniformly distributed throughout its volume.

BACKGROUND

[0003] The production of semi-solidified metal or metal-alloy productsis known, for example, from an article by J. -P. Gabathuler and J.Erling, entitled “Thixocasting: ein modemes Verfahren zur Herstellungvon Formbauteilen” [Thixocasting: A Modem Method for Producing MoldedComponents], which was published in the proceedings of “Aluminium alsLeichtbaustoff in Transport und Verkehr” [Aluminum as a Light BuildingMaterial for Transporting and Traffic], pages 63-77 (ETH Züirich, May27, 1994).

SUMMARY OF THE INVENTION

[0004] An object of the present invention is to prepare a metal ormetal-alloy product from a metal or metal alloy carrier material(hereinafter referred to as “melt”) and an alloy, the product having ahomogeneous distribution of crystallization nuclei throughout its volumeat a point prior to the product being introduced into a mold during thecasting process.

[0005] The present invention achieves this object by introducing anamount of a chosen alloy (in pulverized form) and an amount of a chosenmelt, which is at a temperature above the liquefaction temperature ofthe alloy, into a crystallization vessel, which is heated to below theliquefaction temperature of the alloy, and mixing the melt and the alloytogether in the crystallization vessel by means of electrical and/ormagnetic forces to create the desired product.

[0006] During the introduction of the alloy and the melt into thecrystallization vessel, the pulverized particles of the alloy, whichpreferably is in a powdered form, are immediately enclosed by the meltto form crystallization nuclei, which are then homogeneously distributedwithin the subsequent mixture by means of the electrical and/or magneticforces to form the product.

[0007] In another embodiment of the present invention, the melt isintroduced into the crystallization vessel in the form of a streamflowing between two electrodes, which are supplied with an electricalvoltage. The resulting stream is narrowed, based on the so-called pincheffect, compressed and is already partially split into individual liquiddrops as the melt flows into the crystallization vessel. Thus, thecrystallization vessel is not filled by means of compact and separatestreams (one of melt and one of alloy), but rather by a dispersed streamin which the melt and alloy are partially intermingled. Such a dispersalmeans that the surface area of the resulting stream is clearlyincreased, so that degassing also occurs.

[0008] After the melt has completely flowed into the crystallizationvessel, the melt stream disappears so that the flow of the dispersedproduct stream is also interrupted. For achieving further dispersion,and also for creating an electrical field, an electrical arc isestablished between the product and an electrode within thecrystallization vessel after the introduction of the alloy and the meltinto the crystallization vessel.

[0009] A magnetic field may be generated in the crystallization vesselto promote additional mixing of the product contained therein, and toimprove the uniformity of the distribution of the crystallization nucleitherein. The magnetic field and the electrical field act in differentways on the product, and the particles contained therein, so that themixing effect is enhanced.

[0010] In another embodiment of the present invention, the melt isaspirated into the crystallization vessel, to which a vacuum has beenapplied. By creating a vacuum in the crystallization vessel, thedispersed melt stream is further dispersed into individual drops,increasing the mixing of the alloy with the melt and, thus promoting theformation of crystallization nuclei within the product.

[0011] In a further embodiment of the invention, a protective gas isadded to the melt as it is being fed into the crystallization vessel. Inparticular, the process is farther improved if the protective gas issupplied under pressure. The introduction of the protective gas preventschemical reactions of the alloy with the atmosphere, which couldnegatively affect any subsequent casting process using the product.

[0012] In an apparatus for performing the method, a crystallizationvessel with an inlet for melt and an inlet for alloy in powder form isprovided. The crystallization vessel includes a heating arrangement andis provided in the area of its bottom and its melt inlet with electrodesconnected to a voltage source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Further features, embodiments, and advantages of the presentinvention will become apparent from the following detailed descriptionwith reference to the drawings, wherein:

[0014]FIG. 1 is a cross-sectional view, of a schematic representation ofthe present invention, illustrating the connection between thecrystallization vessel and the furnace;

[0015]FIG. 2 is a cross-sectional view, of a schematic representationillustrating another embodiment of the present invention;

[0016]FIG. 3 is a cross-sectional view, of a schematic representation ofanother embodiment of the present invention illustrating thecrystallization vessel with an added arrangement for receiving theprocessed melt; and

[0017]FIG. 4 represents a nomograph for predicting the thermo-kineticprogress of a product produced by the method of the present invention,specifically the alloy AISI9Cu₃.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Now referring to FIG. 1, in a furnace 10 a melt 11 of a metalalloy, for example AISI 9, is maintained at a temperature greater thanthe liquefaction temperature of the particular alloy. The furnace 10 ismaintained at a vacuum by means of an exhaust device 12.

[0019] The furnace 10 is connected to the crystallization vessel 14 by acasting conduit 13. The crystallization vessel 14 includes a cylinder 15made of an electrically nonconducting material that has a heatconducting capability between 0.20 and 1.5 W/mk. A cover 16, made of anelectrically nonconductive material, closes the top of the cylinder 15.The casting conduit 13 is connected to the cover 16. Preferably, a meltinlet element 17 extends from the casting conduit 13 through the cover16 to allow the melt 11 to flow into the crystallization vessel 14. Themelt inlet element 17 has a conically widening inlet opening and is madeof an electrically conductive material. An aspirating line 18 isconnected to the cover 16 to provide communication between thecrystallization vessel 14 and a suction removal device 19, so that avacuum may be created within the crystallization vessel 14. The cover 16is also provided with a filler neck 20, through which alloy in powderform can be introduced into the crystallization vessel 14. A piston 21,also made of an electrically nonconducting material, is movably insertedinto a bottom of the cylinder 15 to seal a bottom of the crystallizationvessel 14. The cylinder 15, the cover 16 and the piston 21 form achamber for mixing the melt and the alloy into the product. The piston21 travels within a guide cylinder 22 which is connected to thecrystallization vessel 14. A product outlet port (not shown) is integralto the guide cylinder 22 and is used to affect the removal of theproduct from the crystallization vessel 14.

[0020] A heating device 26 is arranged about the crystallization vessel14, to selectively heat and maintain the crystallization vessel 14 at apre-selected temperature. Preferably, the heating device 26 iselectrical and is adjustable. A magnetic coil 27 is arranged about thecrystallization vessel 14. The magnetic coil 27 preferably generates anadjustable magnetic field in the chamber defined by the cylinder 15, thecover 16 and the piston 21 inside the crystallization vessel 14.

[0021] A gate slide 28 is disposed within the casting conduit 13 toregulate flow of the melt from the furnace 10 to the crystallizationvessel 14. A gas supply line 29 is connected to the casting conduit 13,through which a protective gas, for example argon, can be supplied to amelt stream flowing through the casting conduit 13. Preferably, theprotective gas is supplied under overpressure.

[0022] In a preferred embodiment, an electrode 23 is disposed on aninterior of the cylinder 15, preferably near the bottom of the cylinder15 of the crystallization vessel 14. As already mentioned, the meltinlet element 17 is made of an electrically conducting material. Avoltage source 24 is connected to the electrode 23 and the melt inletelement 17 to provide electrical power to both. Preferably, the voltagesource 24 is adjustable, in particular its current strength, by anadjustment device 25.

[0023] The product is prepared by the method discussed as follows. Thefurnace 10 is maintained at a vacuum by operation of the exhaust device12. Preferably, the furnace 10 is maintained at a vacuum between about0.5 mbar and 3 mbar. The melt within the furnace 10 is maintained at atemperature greater than the liquefaction temperature of the alloy.

[0024] The crystallization vessel 14 is heated to a temperature lessthan the liquefaction temperature of the alloy by selectivelycontrolling the heating device 26 attached thereto. Preferably, thecrystallization vessel 14 is maintained at a temperature which is about3% to 50% lower than the liquefaction temperature of the respectivealloy. The suction removal device 19 attached to the crystallizationvessel 14 by the aspirating line 18 creates and maintains a vacuumwithin the crystallization vessel 14. Preferably, the vacuum in thecrystallization vessel 14 is greater than the vacuum maintained in thefurnace 10 to promote the aspirating of the melt from the furnace 10into the crystallization vessel 14.

[0025] Upon opening of the slide gate 28, the melt 11 within the furnace10 is aspirated into the crystallization vessel 14. Protective gas issupplied to the aspirating melt by the gas supply line 29. The vacuumcreated within the crystallization vessel 14 causes the alloy powder tobe aspirated into the crystallization vessel 14 through the filler neck20. The aspirated alloy powder is thus combined with the aspirated meltand is distributed therethrough to form the product.

[0026] A voltage is applied to the electrode 23 and the inlet element 17by the voltage source 24 to establish an electrical current through theproduct within the crystallization vessel 14. Preferably, the current isless than about 10 A. To promote as homogeneous as possible distributionof the crystallization nuclei within the product, radial movement of theproduct within the crystallization vessel 14 is created generating amagnetic field within the interior of the crystallization vessel 14 bythe magnetic coil 27.

[0027] Once the desired amounts of melt and alloy have been introducedinto the crystallization vessel 14, the electric current generatedbetween the electrode 23 and the melt inlet element 17 may betemporarily interrupted. Thereafter an electrical current is establishedtherebetween that preferably has a voltage between about 150 V and 400V, so that an arc is ignited between the electrode and the product, thearc preferably having a current of up to about 1300 A. To prevent adirectional orientation of the crystallization nuclei within theproduct, the magnetic field generated by the magnetic coil 27 isadjusted accordingly and, for example, is continuously increased in thedirection of the fill.

[0028] After the product has been prepared in this manner, the piston 21is lowered, so that the product flows out via the guide cylinder 22 andthe product outlet port for further processing. The product prepared bythe method disclosed herein is suitable for use with all known castingmethods.

[0029] In another preferred embodiment, the electrode 23 is integratedinto the piston 21.

[0030] In another preferred embodiment, illustrated in FIG. 2, thevoltage source 24 is connected to two electrodes 30 and 31 arranged,preferably, in a vertically spaced manner along a portion of thecylinder 15 of the crystallization vessel 14. The voltage source is alsoconnected to a portion of the casting conduit 13. In this embodiment thepiston 21 continuously moves downward while the melt and alloy are fedinto the crystallization vessel, so that the electrodes 30 and 31 aresequentially employed and are switched on and off during the pistonmovement by means of switches 32 and 33.

[0031] In another preferred embodiment, as shown in FIG. 3, the productprepared in the crystallization vessel 14 is passed on to a storage ortransport vessel 34, in which the product is maintained in its preparedstate. The storage vessel 34 is provided with an exhaust device 35, sothat a vacuum may be established therein. A heating device 36 and amagnetic coil 37 are arranged about the storage vessel 34. An electrode38 is disposed within the storage vessel 34. Finally, two opposing walls39, 40 of the storage vessel 34 are comprised of pistons that manipulatethe product as it is stored therein. The storage vessel 34 may forforming the product therein into a more desired configuration forcontinued storage or casting.

[0032] The thermo-kinetic progress of a particular melt/alloy productcan be predicted by means of a nomograph. For example, a nomograph forthe melt/alloy product AISI9Cu₃ is represented in FIG. 4. The amount ofpulverized alloy—added at a grain size of approximately 125 μm toapproximately 400 μm—is entered as percentile amounts (see verticalaxis). The temperature difference (Delta T) in C.° is the differencebetween the casting temperature and the liquefaction temperature of thealloy (see horizontal axis). If the percentage amount of pulverizedalloy added lies within the nomograph range A, it only causes areduction in the temperature of the product, i.e., the product is placedinto a semi-solidified state without the pulverized particles formingcrystallization nuclei. If the percentage amount of pulverized alloy isadded so that the nomograph range B is reached, then the pulverizedparticles act as additional, unmelted crystallization nuclei. Finally,and most desired, if the percentage amount of added pulverized particleslies within the C range of the nomograph, then the two processes willtake place side-by-side, i.e. a reduction of the product temperature andformation of crystallization nuclei because of unmelted particles. It isof course necessary to draw different nomographs for different alloys.It is understood that products of different melts and alloys will havetheir own nomographs.

[0033] It will therefore be readily understood by those persons skilledin the art that the present invention is susceptible of broad utilityand application. Many embodiments and adaptations of the presentinvention other than those herein described, as well as many variations,modifications and equivalent arrangements, will be apparent from orreasonably suggested by the present invention and the foregoingdescription thereof, without departing from the substance or scope ofthe present invention. Accordingly, while the present invention has beendescribed herein in detail in relation to its preferred embodiment, itis to be understood that this disclosure is only illustrative andexemplary of the present invention and is made merely for purposes ofproviding a full and enabling disclosure of the invention. The foregoingdisclosure is not intended or to be construed to limit the presentinvention or otherwise to exclude any such other embodiments,adaptations, variations, modifications and equivalent arrangements, thepresent invention being limited only by the claims appended hereto andthe equivalents thereof.

What is claimed is:
 1. A method for preparing a metal or metal-alloyproduct for a casting process wherein the product is manipulate-able ina semi-solidified state and in which crystallization nuclei aredistributed uniformly therethrough, the product comprising a carriermaterial “melt” and an alloy, the method comprising the steps of: a)feeding the melt into a crystallization vessel, the melt having atemperature greater than a liquefaction temperature of the alloy; b)introducing the alloy into the crystallization vessel simultaneouslywith the introduction of the melt, the alloy being in a pulverized form;c) mixing the melt and the alloy in the crystallization vessel byapplying electrical and magnetic forces thereto; and d) maintaining thecrystallization vessel at a temperature less than the liquefactiontemperature of the alloy throughout the introduction and mixing steps.2. The method according to claim 1, wherein the alloy of the introducingstep is in a powdered form.
 3. The method according to claim 1, furthercomprising the steps: a) liquefying the melt in a furnace before feedingthe melt into the crystallization vessel, the furnace operating at atemperature greater than the liquefaction temperature of the alloy; andb) transporting the melt from the furnace to the crystallization vesselthrough a casting conduit.
 4. The method according to claim 3, whereinthe step of liquefying the melt includes maintaining the furnace at avacuum.
 5. The method according to claim 4, wherein the step ofmaintaining the furnace at a vacuum includes maintaining the furnace ata pressure of about 0.5 mbar to about 3 mbar.
 6. The method according toclaim 3, further comprising the step of regulating transport rate of themelt to the crystallization vessel.
 7. The method according to claim 1,further comprising the step of maintaining the crystallization vessel ata vacuum.
 8. The method according to claim 1, wherein the step ofmaintaining the crystallization vessel at a temperature less than theliquefaction temperature of the alloy includes pre-selecting thetemperature of the crystallization vessel and heating thecrystallization vessel to a pre-selected temperature by a heaterarranged on an exterior of the crystallization vessel.
 9. The methodaccording the claim 1, wherein the step of maintaining thecrystallization vessel at a temperature less than the liquefactiontemperature of the alloy includes maintaining the temperature betweenabout 3% and about 50% lower than the liquefaction temperature of thealloy.
 10. The method according to claim 1, wherein the feeding stepincludes introducing the melt into the crystallization vessel in theform of a stream flowing between at least two electrodes positioned atan inlet of the crystallization vessel, the electrodes being at spacedintervals around the inlet and supplied with electrical power.
 11. Themethod according to claim 1, wherein the mixing step includesestablishing an electrical current between an electrode integral to thecrystallization vessel and an inlet of the crystallization vessel,thereby establishing an electrical arc between the melt and theelectrode.
 12. The method according to claim 1, wherein the mixing stepincludes establishing a magnetic field within the crystallization vesselusing a magnetic coil arranged on an exterior of the crystallizationvessel.
 13. The method according to claim 1, wherein the feeding stepincludes aspirating the melt into the crystallization vessel.
 14. Themethod according to claim 1, wherein the introducing step includesaspirating the alloy into the crystallization vessel.
 15. The methodaccording to claim 3, wherein the feeding step includes aspirating themelt into the crystallization vessel by maintaining the crystallizationvessel at a lower pressure than the furnace, thereby creating a suctionbetween the crystallization vessel and the furnace that acts upon themelt.
 16. The method according to claim 1, wherein the feeding stepincludes supplying a protective gas to the melt.
 17. The methodaccording to claim 16, wherein the supplying step involves using argonas the protective gas.
 18. The method according to claim 1, wherein thefeeding and introducing steps include aspirating the melt and alloy intothe crystallization vessel by maintaining the crystallization vessel ata lower pressure than sources of the melt and the alloy.
 19. Anapparatus for preparing a metal or metal-alloy product for a castingprocess wherein the product is manipulate-able in a semi-solidifiedstate and in which crystallization nuclei are distributed uniformlytherethrough, the product comprising a carrier material “melt” and analloy, the apparatus comprising: a crystallization vessel having a top,a bottom and sides, the top having an alloy inlet and a melt inlet; thebottom having at least one electrode attached thereto; a selectivelyoperable electrical source connected to the at least one electrode andthe melt inlet; and a selectively controllable heating apparatusarranged along an exterior of the crystallization vessel.
 20. Theapparatus according to claim 19, wherein the crystallization vessel isconnected to a means for creating a vacuum therein.
 21. The apparatusaccording to claim 19, wherein a magnetic coil is arranged on theexterior of the crystallization vessel for selectively generating amagnetic field therein.
 22. The apparatus according to claim 19, furthercomprising a furnace connected to the melt inlet of the crystallizationvessel by a casting conduit.
 23. The apparatus according to claim 22,wherein the casting conduit has a protective gas line connected theretofor supplying protective gas to the melt.
 24. An apparatus for preparinga metal or metal-alloy product for a casting process wherein the productis manipulate-able in a semi-solidified state and in whichcrystallization nuclei are distributed uniformly therethrough, theproduct comprising a carrier material “melt” and an alloy, the apparatuscomprising: a crystallization vessel further characterized by: an innersleeve disposed inside the crystallization vessel, the inner sleevebeing generally a hollow cylinder with a bottom, a top and sidesextending therebetween; a cover disposed at and enclosing the top of theinner sleeve; a piston disposed at the bottom of the inner sleeve, thepiston of a size to movably seal the bottom, the inner sleeve, the coverand the piston defining a mixing chamber within the crystallizationvessel; a melt inlet and an alloy inlet extending through the cover forfeeding the melt and the alloy into the mixing chamber; a selectivelyoperable electrical source connected to at least one electrode and anelectrical contact generally opposite the at least one electrode,whereby an electrical current is established between the at least oneelectrode and the contact through the product within the mixing chamber;and a selectively operable heating device arranged along an exterior ofthe sides of the crystallization vessel for selectively heating theproduct within the mixing chamber.
 25. The apparatus according to claim24, wherein the at least one electrode is a single electrode disposed onan inner surface of the inner sleeve, the contact being the melt inlet.26. The apparatus according to claim 24, wherein the at least oneelectrode is attached to the piston, the contact being the melt inlet.27. The apparatus according to claim 24, wherein a suction device isconnected to the cover to selectively maintain the mixing chamber at avacuum.
 28. The apparatus according to claim 24, wherein a magnetic coilis arranged on an exterior of the crystallization vessel for selectivelygenerating a magnetic field within the mixing chamber.
 29. The apparatusaccording to claim 24, further comprising a furnace connected to themelt inlet by a casting conduit for feeding the melt into the mixingchamber, the furnace having an operating temperature greater than aliquefaction temperature of the alloy.
 30. The apparatus according toclaim 29, wherein the casting conduit has a protective gas lineconnected thereto for supplying protective gas to the melt flowingthrough the casting conduit.
 31. The apparatus according to claim 29,wherein the at least one electrode comprises at least two electrodesdisposed on an inner surface of the inner sleeve, the electrodes atvertically spaced intervals thereon, and the contact is a portion of thecasting conduit.
 32. The apparatus according to claim 24 wherein themelt inlet is generally conical with a wide base extending upwardly to anarrow top, the melt inlet being made of electrically conductingmaterial.
 33. The apparatus according to claim 24, wherein the innersleeve, the piston, and the cover are made of non-electricallyconductive material.
 34. The apparatus according to claim 24, whereinthe inner sleeve is made of thermally conductive material.
 35. Theapparatus according to claim 24, wherein the inner sleeve has a thermalconductivity of about 0.20 W/mK to about 1.5 W/mK.
 36. The apparatusaccording to claim 24, wherein the heating device operates at apre-selected temperature.
 37. The apparatus according to claim 24,wherein the heating device maintains the chamber at a temperature lessthan a liquefaction temperature of the alloy introduced therein.
 38. Theapparatus according to claim 29, wherein the casting conduit includes agate for controlling flow of the melt from the furnace to thecrystallization vessel.
 39. The apparatus according to claim 29, whereinthe furnace is maintained at a vacuum.
 40. The apparatus according toclaim 29, wherein the furnace is maintained at about 0.5 mbar to about 3mbar.
 41. The apparatus according to claim 29, wherein the mixingchamber of the crystallization vessel is maintained at a greater vacuumrelative to the furnace.
 42. The apparatus according to claim 24,wherein the selectively operable electrical source provides a currentbetween about 0 A and about 1300 A through the product within the mixingchamber.
 43. The apparatus according to claim 24, wherein the pistonmoves within a cylindrical guide extending from the bottom of the innersleeve, the guide including a product outlet for removal of the productfrom the mixing chamber, wherein removal of the piston from the bottomof the inner sleeve exposes product within the mixing chamber to theguide and the product outlet.
 44. The apparatus according to claim 24,further comprising a storage vessel connected to a product outlet portin the crystallization vessel, the storage vessel having an exhaustdevice attached thereto for maintaining a vacuum therein, a selectivelyoperable heating device arranged on an exterior of the storage vessel tomaintain the storage vessel at a selected temperature, and a selectivelyoperable magnetic coil arranged on the exterior of the storage vesselfor generating a magnetic field therein.
 45. The apparatus according toclaim 44, wherein at least two opposing walls of the storage vesselcomprise pistons, the pistons being moveable for manipulation of theproduct therein.